THE MECHANICAL PROPERTIES OF STAINLESS STEEL
How these are determined, and the factors which influence their values
by David P. Rowlands, BSC Eng (Witwatersrand), MIM, CENG.
Steel relative to the more common materials of construction.
Stainless Steels are primarily utilized on
account of their corrosion resistance.
However, the scope of excellent
mechanical properties offered by the
various classifications and grades within
the FAMILY of STAINLESS STEEL
render them extremely versatile materials.
MECHANICAL
PROPERTIES
In this article the significance of these
different properties will be covered with a
view to enabling a fuller and better
appreciation of the vast amount of published data which is available.
(1) Yield Strength and
Tensile Strength
Throughout
this
article
Nominal,
Resentative or Typical values will be
used to illustrate the different properties
being discussed (unless noted otherwise)
and will be referred to as nominal values.
The nominal values are those which are
the norm for the various properties, but it
is STRESSED that these must NOT be
regarded as minimum, nor for some properties maximum values for specification
purposes.
As the increasing tensile load is
applied, a diagram (graph) is plotted to
show the progressive relationship
between the STRESS and STRAIN.
The mechanical properties are a
measure of the metals response to an
applied force.
The most common mechanical property
used for comparison, reference and
design purposes is that of strength, both
the Yield Strength and the Tensile
Strength.
To determine both these strengths, standard test specimens, which are
schematically shown in Fig 1, are
machined from the material and then
subjected to an increasing measured load
in tension until rupture (breakage) occurs.
The nominal values inevitably vary,
depending on the reference or source of
the publication, eg Grade 304 Hot Rolled
and Annealed Plate at Room Tempera
ture.
Tensile
Yield
Strength
Strength
Source "A" 565 MPa
241 MPa
Source "B" 600 MPa
310 MPa
Specification 500 MPa
205 MPa
ASTM A240
(min)
(min)
As may be seen from this example, it is
usually the case that the specified values
are of a lower (more "conservative") value
than the nominal values.
Therefore, if guaranteed values are
required, reference must be made to the
actual Code or Specification. Different
Specification and Codes stipulate different values, and this can lead to either
less or more cost-effective utilization of
material, a factor which is of importance
considering the higher price of Stainless
The Stress-Strain Curve is typified by various
regions. Refer to Fig 2.
• From O to E the strain produced is elastic.
On removal of the stress the specimen (or the
material in actual use)
will revert to its original dimensions. The
stress value corresponding to E is
termed the Elastic Limit.
• As the stress is raised above the Elastic Limit (E), the material will start to
deform in a plastic manner, ie the material undergoes a permanent strain,
deformation or increased dimensions.
Many metals {mild steel included) show
a pronounced Yield Point, as indicated in
Fig 3. This occurs at slightly higher
stress levels than the elastic limit. At the
Yield Point the material shows a sudden
increase of strain for no increase in
stress. For such steels this is reported as
the Yield Strength or the Yield Stress.
• With further tensile loading the Dlasamount of increased stress required to
induce permanent strain decreases, as
indicated by the flattening of the curve
between Y and T in Fig 2. In this region
the deformation is uniform within the
reduced cross-section of the tensile test
specimen, until the resistance of the
material for stress to produce permanent
strain reaches a maximum at T.
The stress corresponding to T, ie the
maximum load divided by the original
cross-sectional area of the specimen, is
taken and reported as the Tensile
Strength. This value is also sometimes
termed the Ultimate Strength or the Ultimate Tensile Strength (UTS).
• After this the plastic deformation be
comes localized in a short length of the
test specimen. Rapid decrease of the
cross-sectional area occurs (NECKING),
and therefore the stress required to pro
duce strain decreases, as shown in Fig 2
by the falling portion of the curve TF.
• The final fracture (breakage) of the
specimen occurs at F.
(2)
Elongation and
Reduction of Area (RA)
Stainless Steels do not typically show
this clearly demarked Yield Point, and
the change from elastic behaviour to
permanent plastic deformation is not
usually easy to detect.
This has led to the Yield Strength of
Stainless Steels being reported using
different terminology, a factor which
often causes confusion. In the initial
stages of plastic deformation, only a
small amount of permanent strain occurs
for relatively large increases in stress.
The Yield Strength for Stainless Steels
is usually taken as the stress which will
produce a 0,2% permanent strain (offset).
Referring to Fig 2, this is obtained by a
line drawn parallel to OE from 0,2%
strain to intersect the curve at Y. The
stress value corresponding to Y is taken
as the Yield Strength.
The Yield Strength for Stainless Steels is
therefore reported as 0,2% Proof Stress
(Rp 0,2), Proof Stress (0,2% offset),
Proof Stress (0,2% strain), Yield
Strength (offset 0,2%), 0,2% Yield
Strength, etc. All these different terms
may be taken to be equivalent.
(Note: Occasionally the Yield Strength is
determined and reported at different
amounts of strain or offset eg 0,1% or
0,5%. This factor should be taken into
account when comparing reported
figures or if designing to minimum
specified values).
Concurrent with the determination of the
Yield and Tensile Strengths, the
mechanical properties of Elongation and
Reduction of Area (RA) are also
determined.
ELONGATION is determined as follows:
Before the tensile test specimens are
subjected to the load, a standard length is
marked on the reduced section of the test
specimen. This is known as the Gauge
(Gage) Length, and is indicated by "G" in
Figs 1(a) and 1(b).
Different specifications lay down different
Gauge Lengths, eg 50 mm (2"), 200 mm
(8") or sometimes as a formula: Gauge
Length - 5,65 √So (ie. 5,65 x Square Root
of the Original cross-sectional area).
Eg for a round tensile specimen of 12,5
mm diameter in the reduced section the
gauge length would be
5,65 √ π x (6,25)2 = 62,6 mm (2,5").
After fracture the two pieces are carefully
put together and the increased distance
between the marks is taken.
The Elongation is calculated by:
(Distance between marks after fractureOriginal Gauge Length) x 100
Original Gauge Length and is thus
reported as a percentage, with the gauge
length specified.
eg Elongation in 50 mm = 28%.
Note: Elongation taken over different
gauge lengths can vary. The elongations
determined on shorter gauge lengths,
because of the influence of necking,
tend to show slightly higher values than
the elongation measured in longer gauge
lengths. This should be taken into account
when comparing reported val-ues.
REDUCTION OF AREA (RA) is determined as follows:
After fracture the two pieces of the
specimen are carefully fitted together, and
the average diameter, or the width and
thickness, of the smallest cross section to
which the specimen has been reduced by
necking is measured.
The RA is calculated by:
(Original cross sectional area
- Smallest cross sectional area
_______ after fracture) _______ x 100
Original cross sectional area
and is thus reported as a percentage. eg.
RA = 55%.
Both Elongation and RA are a measure of
the DUCTILITY of the material, ie the
ability of the material to deform in a plastic manner without fracturing.
Elongation is the property most often used
in this respect. RA suffers from the
drawback that even at relatively low levels
of ductility the RA has values of ±50%. As
the ductility increases RA rapidly
increases to high values ±70-75%, which
renders it insensitive to a meaningful
assessment of ductility.
(3) Hardness
Hardness is an often a reported mechanical property, and is useful as a means of
giving an indication of the tensile strength,
and as a non-destructive test for checking
heat treatment and the sorting of material.
Hardness is determined by measuring the
resistance of the material to penetra-tion
(indentation).
-- Brinell Test: A hardened steel ball indentor (10 mm diameter) is forced into
the material by a standard load. The
diameter of the impression gives, from
tables, the Brinnel Hard-ness Number
(BHN).
-- Rockwell Test: Either a hardened steel
ball (Rockwell B - HRB) or a diamond
brale (Rockwell C - HRC) is forced
into the material by standard applied
loads. The depth of penetra-tion is
used to give a Rockwell number
directly from the scale on the
equipment. HRB is used for soft
materials, and HRC for hard mate
rials.
-- Other Hardness tests are also sometimes used.
Vickers
Test:
Vickers
Pyramid
Number (VPN), usually only used in
the laboratory.
Shore Test: Measures the rebound of
a hardened ball up a standard tube.
Used when no indentation can be
tolerated.
Some indicative comparisons (Relative
Basis of Hardness).
Very Hard
62HRC 688 BHN
Hard
48 HRC 455 BHN
Medium Hardness
30 HRC 286 BHN
Soft
90HRB 185 BHN
Very Soft
72 HRB 130 BHN
(Note: Brinell Hardness Tests are not
carried out on materials of over 400-450
BHN, as the hardened ball indentor would
itself tend to suffer distortion).
(4) Toughness
Toughness is the capacity of a material to
yield plastically under conditions of highly
localized stress. It is an important
engineering property.
they cannot be converted to energy
values which may be utilized in
engineering calculations.
Impact testing may also be used to
assess the effect of lower temperatures
on a material. Impact specimens are
uniformly heated or cooled to various
temperatures, and quickly tested before
they lose or gain heat from the surroundings.
A plot of the energy absorbed is made
against the testing temperature of the
specimens. The Ductile to Brittle Transition Temperature (DBTT) is taken at the
temperature at which the slope of the
curve changes. This is shown schematically in Fig 5.
Most steels suffer from a loss of toughness as the temperature drops to freezing
point (0°C) and below. The actual Impact
Strength at the DBTT varies for different
steels. As a general rule if this value is
less than 35 J, the material is described
as "brittle". This does not necessarily
preclude the use of the material.
However, it does indicate that the
material must be used with due consideration. Applications which involve any
impact loads or dynamic stress are very
susceptible to the presence of defeats or
imperfections, and are best avoided.
Applications involving static or constant
loads are generally acceptable.
Some'Steels exhibit Impact Strengths in
It is determined using a dynamic test. A
standard specimen which is notched to
localize the stress, is struck by a single
blow from a swinging pendulum. The
energy absorbed in deforming or breaking the specimen is measured by the loss
of energy in the pendulum.
Tough materials absorb a lot of energy,
whereas brittle materials absorb little
energy.
The most often used test is the CHARPY
"V" NOTCH IMPACT TEST. Similar tests
are and have been used viz The Charpy
Keyhole, and Izod Test.
Fig 4 schematically illustrates the Charpy
"V" Notch Impact Test.
Impact Values are reported as the energy
absorbed, with the type of test and the
temperature noted. The units are Joules
(J).
Eg Impact Strength: Charpy "V" (20°C) =
60J.
The values determined are qualitative
comparisons and although often reported,
or specified as acceptance criteria,
excess of 35 J at the DBTT and even at
lower temperatures, and therefore are
considered as "tough" at all temperatures.
General Note: It is usual to take test
specimens in the direction of rolling or
extension (termed Longitudinal). Thus
strain or fracture takes place at right
angles to any directionality which may
exist in the material. Such longitudinal
test specimens generally exhibit higher
values compared to values obtained from
test specimens taken across the rolling
direction (termed Transverse), because
any directionality which may exist now
lies across the cross-section of the test
specimen, which tends to initiate and
cause failure at lower levels of stress.
AUSTENITIC STAINLESS
STEELS
Table 1: Nominal room temperature Yield and Tensile Strengths for annealed
Austenitic Stainless Steels.
Yield Strength and
Tensile Strength
AISI Type
301
304
304L
305
309S
31 OS
316
316L
Yield Strength (0,2%
offset) MPa
275
290
270
262
310
310
290
290
Nominal
room
temperature
Yield
Strengths (0,2% offset), and Tensile
Strengths for some annealed Austenitic
Stainless Steels are given in Table 1.
Tensile Strength MPa
755
580
560
585
620
655
580
560
Compared to normal structural plain
carbon mild steel (Yield Strength 270
MPa, Tensile Strength 465 MPa), it may
be seen that the Austenitic Stainless
Steels have superior properties.
Table 2: Indicative response to cold working different grades of Austenitic
Stainless Steels to varying degrees.
• Austenitic Stainless Steels further show
a marked response to cold working. Such
cold working significantly increases both
the Yield and Tensile Strengths.
The degree to which work-hardening
affects the strength levels depends on the
chemical composition of the different
Austenitic Stainless Steels, particularly
with regard to the content of the elements
which stabilize the Austenitic crystal
structure (especially the Nickel (Ni)
content).
For example in Table 2 Grade 301, which
does not have a composition necessary
to fully stabilize the Austenitic structure, is
compared with Grade 304 and Grade 310
which may be considered to have fully
stabilized Austenitic structures at normal
room temperature. Lower response to
work-hardening of these latter grades is
indicated.
The Austenitic Stainless Steels therefore
have the ability to be work-hardened to
very high strength levels, exceeding to a
significant degree those obtainable from
conventional structural steels and heat
treated tow alloy steels.
High strength Austenitic Stainless Steel
sheet and strip can be cold rolled to effect
sufficient reduction in thickness, thus
developing the required high strength
properties. No subsequent annealing is
carried out. These higher strengths are
defined according to temper. The
process is referred to as temper rolling
to differentiate it from, and prevent any
confusion by simply referring to it as "cold
rolling".
The
strength
properties
associated with the various tempers is
given in Table 3.
Table 3: Strength levels for
various Temper Rolled
sheet/strip.
Grade
301
304
310
% Cold Work
Tensile Strength
MPa
10% Cold Work
585
1035
30% Cold Work
1035
1275
50% Cold Work
1310
1445
10% Cold Work
480
685
30% Cold Work
825
860
50% Cold Work
1000
1100
10% Cold Work
470
744
30% Cold Work
854
965
50% Cold Work
1010
1145
Such temper rolling is usually confined to
Grade 301 because of its greater
response, and other grades of Austenitic
Stainless Steel are not readily obtainable
in this condition.
Further, high strength wire is also produced by the same principle. Different
tempers are also used:
-- Annealed Temper: Soft wire. No
drawing takes place after the last
annealing treatment.
-- Soft Temper: One single light draft
draw after the last annealing treatment, to produce properties below a
specified maximum.
-- Intermediate Temper: A draw of progressive drafts after annealing to
produce minimum specified strengths.
-- Spring Temper: A draw of several
drafts to produce the high tensile
spring wire requirements.
The tensile strengths obtainable in the
Spring Temper condition are very high,
especially for small diameter wires.
Several standard grades are contained in
those normally drawn to wire, viz: 302,
304, 305, 316, 321, etc. Nominal tensile
strength levels are indicated in Table 4.
Yield Strength
(0,2% offset)
MPa(min)
Tensile
Strength
MPa(min)
1
/4 Hard
515
860
1/2 Hard
760
1035
Table4: Tensile Strength levels
obtainable for cold drawn wire (Grade
304)
Wire
Tensile strength range
Diameter
(MPa)
0,46-0,51 mm
2070-2275
3/4 Hard
930
1205
0,94-1,04 mm
1895-2095
Full Hard
965
1275
2,41-2,67 mm
1600-1805
Temper
Yield Strength (0,2%
offset) MPa
- As the temperature increases to elevated
(up to 500°C) and high (above 500°C) levels,
the strength properties decrease. This is
simply because the atoms move around more
easily, due to the cohesive bonds between
them relaxing as the temperature increases.
Nevertheless Austenitic Stainless Steels
possess useful elevated and high temperature
strengths.
Elevated and high temperature strengths are
reported in two different ways, viz
+ SHORT TIME PROPERTIES. The tensile
test specimen is heated to the required
temperature of the test, and actually tested
at this temperature to obtain the StressStrain curve and the resultant Yield and
Tensile Strengths.
The effect of elevated and high temperatures
on both the Yield and Tensile Strengths in
MPa, is taken on a Short Time basis, as
indicated in Table 5. In general terms it may
be seen that the "L" Grades suffer a greater
loss of properties with a rise in temperature,
particularly with regard to their tensile
strengths.
Against the values given in Table 5, it is
interesting to note the maximum allowable
stresses in tension for Austenitic Stainless
Steels in Unfired Pressure Vessels (Section
VIII Boiler and Pressure Vessel Code,
American Society for Mechanical Engineers).
As an example the allowable stresses for 304
and 304L are given in Table 6.
+ LONG TIME PROPERTIES. At high
temperatures (ie in excess of
500ºC) the strain is dependent
on the applied stress and time.
The metal undergoes a continuous slow
deformation, which is realistically termed
CREEP. Creep can occur at stresses
below the Short Time Yield Strength.
Table 5: Indicative short time elevated and high temperature Yield and Tensile
Strengths for various Austenitic Stainless Steels.
(Note: YS = Yield Strength, TS = Tensile Strength both in MPa)
Temp°C
304
304L
321
316L
316
347
YS
TS
YS
TS
YS
TS
YS
TS
YS
TS YS
TS
150
191
465
180
431
205
510
176
450
156
475
224
480
260
166
445
152
409
173
500
149
435
131
468
198
437
370
150
445
140
400
157
500
134
435
121
468
182
426
1. Primary Creep over a relatively short
time during which the creep rate de
creases.
480
137
428
130
382
152
475
123
415
116
468
176
426
595
125
365
116
327
144
407 109
360
112
400
173
402
2. Secondary Creep at a constant rate
over an extended period at a minimum creep rate.
705
112
266
=
245
134
298
=
272
102
276
161
324
815
79
145
=
142
112
172
=
168
95
141
113
168
There are three stages of creep as indicated in the schematic diagram in Fig 6.
3. Tertiary Creep over a relatively short
time during which the creep rate increases and the metal finally fractures.
Long Time Strength Properties
generally expressed in two ways: -
are
-- The Creep Stress is that which will
cause a specific rate of deformation in
a given time (ie within the Second-ary
Creep Range) at a specific temperature.
-- The Rupture Stress is that which will
cause rupture (ie encompasses all
stages of Creep) in a given time and
at a specific temperature.
The values are usually presented in a
diagrammatic form. A typical Creep Rate
Rupture Time diagram for Grade 304
Austenitic Stainless Steel is shown in Fig
7.
• To obtain Rupture Time; At a constant temperature of 734°C (1 350°F),
if the stress is 69 MPa (10 000 psi) it
indicates that the material will rupture in approximately 250 hours.
However, if the stress is reduced to 27
MPa (4 000 psi) the material will take
22 000 hours to rupture.
• To obtain Creep Rate: At a constant
temperature of 734°C (1 350°F), if the
stress is 26 MPa (3 750 psi) the Creep
Rate is approximately 0,0001% per
hour. Therefore in 10 000 hours an
original dimension will increase by
(10 000 x 0,0001%) = 1,0% ie an original dimension of 100 mm would have
become 101 mm.
In 25 000 hours an original dimension
of 100 mm would have theoretically
become 102,5 mm - BUT this cannot
occur because rupture would have
taken place at 22 000 hours as seen
above.
If the stress is reduced to 14 MPa (2
000 psi) the Creep Rate is approximately 0,00002% per hour. Therefore
in 10 000 hours an original dimension
will have increased by 0,2% ie. 100
mm would become 100,2 mm.
Table 6: Indicative maximum allowable stresses in tension for use in Unfired Pressure Vessels.
Allowable stress in
Steel grade
304
304L
MPa for temperatures
150°C
260 °C
370 °C
480 °C
595 °C
705 °C
815°C
103
86
74
65
52
17
5
90
67
58,5
=
=
=
=
At any temperature the maximum stress
for the maximum time is given by the
intersection of the Rupture Time and the
Creep Rate.
For example at 734°C the maximum is
approximately 24 MPa (3 500 psi) which
will give a Rupture Time of 60 000 hours.
The Creep Rate is approximately
0,00006% per hour, which for 60 000
hours is 3,6%, ie 100 mm original length
would increase to 103,6 mm. Naturally
such an increase in dimensions may not
be tolerable in the application. Long Time
Creep values are sometimes presented in
tabular form as given in Table 7.
The interdependence of Creep Rate and
Rupture Time should be noted.
Thus, the reason for the lower allowable
stress values for Pressure Vessels (Table
6) may be appreciated in comparing the
values in Tables 5,6 and 7.
* Austenitic Stainless Steels increase
their strength progressively as the temperature is lowered, including the
cryogenic temperatures of liquid gases,
viz 200°C below freezing point.
The underlying reasons for this are complex, and beyond the scope of this article.
It is sufficient to say that those Austenitic
Stainless Steels which have higher Nickel
(Ni) and/or Nitrogen (N) content
(elements which have a strong influence
on stabilising the Austenitic crystal
structure), ie stable alloys, tend to have a
less pronounced increase in Tensile
Strength, but a marked increase in the
0,2% offset Yield Strength as the
temperature decreases.
Those alloys which have low Ni and N
contents undergo change to the crystal
structure as the temperature drops, ie
metastable alloys. Such alloys have a
more pronounced increase in the Tensile
Strength, but only a slight increase in the
0,2% offset Yield Strength.
Indicative values are given in Table 8 for
304 (metastable) 304N (stabilised with N)
and 310 (stabilised as a result of its high
Ni content).
Ductility. Annealed Austenitic Stainless
Steels have excellent Elongation values
of typically 50-60% and higher.
They therefore possess a superior ability
to be cold formed, pressed, drawn and
spun into deep shapes.
Cold working does bring about a decrease in the Ductility. Elongation of ±
20% are typical for material which has
undergone 30% cold work - still a very
acceptable Ductility by normal engineering standards.
At sub-zero temperatures the Elongation
decreases only slightly, giving a typical
Elongation value of 40-50%.
Hardness. In the annealed condition
typical Hardness is 150-160 HBN. Small
amounts of cold work can rapidly
increase the Hardness up to levels of
Table 7: Indicative maximum permissible stress values for different Creep Rates for
304 Stainless Steel.
Temperature
538 °C
648 °C
734 °C
815°C
Stress for 1% in 10 000 hours
138. MPa
56 MPa
25 MPa
19 MPa
Stress for 1% in 100 000 hours
76 MPa
27,5 MPa
12 MPa
9 MPa
Table 8: Indicative 0,2% offset Yield Strength and Tensile Strengths for different
Austenitic Stainless Steels at temperatures below freezing point.
304
304N
Test
Temperature
°C
0,2% YS
MPa
TS
MPa
0,2% YS
MPa
- 50
236
1101
495
-100
223
1280
615
-140
245
1365
-196
232
1610
± 250 HBN. Further cold work results in
a slower increase in Hardness. Spring
Temper wires and Grade 301 cold rolled
to Full Hard Temper have Hardnesses of
the order of 340-380 HBN.
Toughness. The annealed Austenitic Stainless Steels have excellent
Toughness, with Charpy V (Room Temperature) typically in excess of 165 J.
Charpy V values at sub-zero temperatures do decrease, but even at temperatures as low as 196°C below zero are
typically 90-120 J, ie not approaching
values which are considered as "brittle".
Cold work also reduces the impact
strength values. The relative reduction
depends on the amount of cold work.
Even severely cold work Austenitic
Stainless Steels retain a considerable
amount of Toughness, and the decrease
will ordinarily never approach a brittle
condition.
Only under two circumstances may
Austenitic Stainless Steels exhibit low
Toughness:
1. If the composition is such that when
heated to the high temperatures of
600°-850°C for extended periods, the
brittle phase called Sigma develops
within the crystal structure. The em
brittling effect of Sigma is not dele
terious at these high temperatures,
but seriously impairs the Toughness
of steels containing it are cooled and
tested at room temperature, or if
plant operating at high temperatures
cools to ordinary temperatures dur
ing shut down.
2. If Austenitic Stainless Steels are first
cold worked to a significant degree
and then heated in the sensitization
temperature range of 550°-850°C for
sufficient time to cause carbide preci
pitation, such sensitized steels will
suffer loss of Toughness at tempera
tures lower than 100°C below zero.
From the foregoing it may be seen that
Austenitic Stainless Steels have exceedingly good low temperature mechanical
TS
MPa
310
0,2% YS
MPa
TS
MPa
392
848
1110
466
950
665
1345
557
972
850
1620
671
1138
940
properties. It is for this reason that they
are used virtually exclusively for the
manufacture of vessels to contain liquid
gases at cryogenic temperatures, Grade
304N being preferred on account of its
superior strength properties.
Fatigue. If metals are subject to
repeated fluctuating (reversing) loads at
stresses below the Tensile Strength,, a
fatigue crack can initiate in the material,
which then with increasing cycles of
loading propagates until final failure by
fracture occurs.
The resistance to Fatigue at various
stress levels is therefore required for
many engineering applications.
Fatigue Resistance is assessed by subjecting polished specimens to simple
forms of repeated cycles of a fixed
reversing stress until fracture occurs.
The higher the stress, the lower the
number of stress cycles required to
bring about failure.
The results so obtained at different
stress levels against the corresponding
number of cycles to final failure are plotted to give a curve termed an S-N CURVE
(ie Stress-Number of Cycles), as shown
schematically Fig 8.
as the number of stress cycles
increases. The stress to produce failure
in a specific (large) number of stress
cycles is termed the FATIGUE
STRENGTH.
In practical terms, however, the
progressive
decrease
in
stress
becomes so small at large number of
cycles that, provided the number of
cycles is large enough, the FATIGUE
STRENGTH may be taken as a
"Fatigue Limit".
(Note: The term ENDURANCE LIMIT is
also used correctly as the Fatigue Limit,
but often incorrectly as the Fatigue
Strength).
Austenitic Stainless Steels have
nominal Fatigue Strengths as shown in
Table 9.
Table 9: Nominal Fatigue Strength
for various annealed Austenitic
Stainless Steels.
Steel grades
Fatigue Strength (MPa)
301
245
304
245
316
265
321
260
Cold working which results in
increased
Tensile
and
Yield
Strengths, also increases the Fatigue
Strengths. Similarly the increased
Tensile and Yield Strengths at subzero temperatures also increases the
Fatigue Strength.
At these higher strength levels,
however, any surface defects or
imperfections adversely affect the
Fatigue Strength, lowering it to a half
or a third of the value obtained with
perfect surface condition.
FERRITIC STAINLESS
STEELS
Yield Strength and
Tensile Strength
Nominal room temperature Yield
Strengths (0,2% offset) and Tensile
Strengths
for
annealed
Ferritic
Stainless Steels are given in Table 10.
* Ferritic Stainless Steels show little
response to cold working. An increase
in both the Yield and Tensile
Strengths takes place, but to a minor
degree only.
* Ferritic Stainless Steels have useful
elevated temperature strengths, but
these strengths tend to fall off rapidly
at high temperatures.
Table 10: Nominal room temperature Yield and Tensile Strengths for annealed
standard and proprietary alloy Ferritic Stainless Steels.
Steel Grade
(•) Proprietary Alloy Grades
Yield Strength
(0,2% offset)
(MPa)
Tensile Strength
(MPa)
430
345
510
446
345
550
(•) 18-2 Cr Mo Super Ferritic
340
515
(•) 26-1 Cr Mo Super Ferritic
345
480
(•) 3CR12
320
500
Table 11: Nominal short time elevated and high temperature Tensile Strengths
for standard and proprietary alloy Ferritic Stainless Steels.
Tensile strength (MPa)
Temperature °C
430
446
18-2 Super Ferritic
3CR12
200
465
580
480
420
400
395
550
450
360
600
165
240
=
160
700
89
110
=
=
800
45
55
Some nominal values are given in Table
11.
In general Ferritic Stainless Steels are
not recommended for use at high temperatures, as their creep strengths at
temperatures above 450°C are low. The
standard Ferritic Stainless Steels are
used at high temperatures on account of
their oxidation (scaling) resistance
(especially Grade 446), but should not be
considered for load bearing components under these conditions.
Fatigue. Nominal Fatigue Strengths
for Ferritic Stainless Steels are of the
order of 310-330 MPa.
Ductility. Ferritic Stainless Steels
have nominal Elongation values as indicated below:
430
446
18-2 Super Ferritic
26-1 Super Ferritic
3CR12
=
=
=
=
=
25%
20%
30%
28%
22%
They therefore have a Ductility equivalent to the usual plain carbon mild steels,
and are therefore suitable for cold forming operations of a moderate degree.
(Note: In this respect a Deep Drawing
Quality (DDQ) Grade 430 has been
developed. Whereas it has drawing
properties superior to standard 430,
these do not approach those exhibited
by the Austenitic grades).
Hardness. Ferritic Stainless Steels
in the annealed condition have a nominal hardness of 165 HBN.
They are non-hardenable alloys, neither
by heat treatment nor by cold work.
=
Toughness. The Impact Strengths of
Ferritic Stainless Steels vary according
to the chemical composition, particularly with respect to the Ductile to Brittle Transition Temperature (DBTT).
Grade 446, for example, has a DBTT of
120°C ie it may be considered of low
Toughness at room temperature. The
standard Ferritic Stainless Steels are
tough at room temperature, and in general their DBTT is between 20°C and 0°C,
thus below 0°C may be considered of
low Toughness.
The Super Ferritic Stainless Steels and
3CR12, with specially controlled chemical compositions, particularly with
respect to specified low levels of both
Carbon (C) and Nitrogen (N), have a
DBTT of normally between 0°C and 20°C
below freezing point.
In general, therefore, all the Ferritic
Stainless Steels are not suitable for use
at low temperatures approaching or
below freezing point.
Note: It is stressed that welding has a
marked effect on the toughness of standard Ferritic Stainless Steels within the
weld zone, due to changes to the crystal
structure brought about in the Heat Affected Zone (HAZ).
MARTENSITIC STAINLESS
STEELS
Martensitic Stainless Steels are usually
supplied in the annealed (soft) condition
for ease of machining. As such they have
mechanical properties similar to the
Fer-
ritic Stainless Steel, because in this condition
they do possess a Ferritic structure.
To develop their attainable mechanical
properties (and their corrosion resistance)
they require to be heat treated by quenching
and tempering, (also sometimes referred to
as hardening and stress relieving). This
involves:1.
2.
3.
Heating the steel to within a specified
high temperature range for sufficient
time to ensure the uniform attainment of
this temperature throughout the crosssection. At this temperature the crystal
structure becomes fully Austenitic, and
therefore it is termed the austenizin
temperature,
or
sometimes
the
hardening tempera-ture.
Rapidly cooling (quenching) the steel
from this high temperature, usually in
oil. (Faster quenching in water is seldom used-slower quenching by simply free cooling in air may sometimes
be used). This quenching produces
within the steel the Martensitic crystal structure, which is highly stressed,
hard and strong, but which has low
Ductility and Toughness.
Immediately tempering the quenched
steel by re-heating to a temperature
necessary to produce the desired
combination of Strength and Hardness, Ductility and Toughness.
The modification and combination of
properties attained by tempering is
schematically indicated in Fig 9.
Grade of
Steel
Tempering
Temperature °C
Yield
Strength
(0,2% offset)
MPa
Tensile
Strength
MPa
Elongation
in 50 mm
%
Hardness
Izod "V"
Notch J
410
200
315
425
538
648
760
1000
965
1035
790
585
415
1310
1240
1345
1000
758
620
15
15
17
20
23
30
41HRC
39HRC
41HRC
31HRC
97HRB
89HRB
47
47
102
136
420
200
315
425
538
648
1380
1345
1380
1000
585
1755
1725
1755
1170
790
10
10
10
15
20
48HRC
14
431
200
315
425
538
648
1070
1035
1070
895
655
1415
1345
1415
1035
860
15
15
15
18
20
43HRC
41HRC
43HRC
34HRC
24HRC
41
61
68
440A
440B
440C
315
315
315
1655
1860
1895
1795
1930
1965
53
2
51HRC
55HRC
57HRC
5
4
3
*Note: Historically the Izod Impact Test was used. These figures are still quoted in most
references. It is not possible to convert results from one method of impact testing to
another.
sectional area, the quenching medium
(rate) used, and the austenitizing (hardening) temperature.
From both Fig 9 and Table 12 it may be
noted that the Strength and Hardness of
these steels is maintained up to a tempering temperature of approximately
425°C. Tempering is usually effected at a
temperature between 260°-300°C, which
gives
satisfactory
Elongation
and
maximum Toughness for the high
Strength and Hardness maintained at
these lower tempering temperatures.
Note, in particular, the typical drop in
Toughness between 375°C and 600°C.
Tempering in this range is not recommended as low and erratic impact properties are obtained.
Tempering above 600°C results in Elongation and Toughness improving markedly, but with a major sacrifice of
Strength and Hardness. The high carbon
grades (440 A, B and C) are mainly used
at the high strength and hardness levels
due to their hardness, abrasion and wear
resistance, eg as pivot pins, dental and
surgical instruments, cutlery, hardened
balls, needle valves, bushing and
bearings.
Nominal values for the Yield Strength
(0,2% offset), Tensile Strength, Elongation, Hardness and Toughness for some
quenched and tempered Martensitic
Stainless Steels are given in Table 12.
The effect of different chemical compositions should be noted, particularly the
effect of Carbon (C) content (medium to
high levels in Grade 420, and Grades 440
A, B, C respectively). The mechanical
properties actually obtained from the
various grades in practice will depend
on the chemical composition, the cross
At these levels, the associated mechanical properties of Elongation and Toughness are relatively inferior. This must be
taken into consideration if they are used
in engineering applications involving
shock or impact loading, dynamic conditions, etc.
The Fatigue properties of Martensitic
Stainless Steels may be correlated to the
quenched and tempered Tensile Strength
level. As a nominal value, the Fatigue
Limit may be taken as approximately 45%
of the Tensile Strength. At such high
levels, fatigue performance is adversely
lowered by the presence of
any surface imperfections or defects.
The quenched and tempered steels may
be used at elevated temperatures up to
25°-30°C below their tempering temperature. If the tempering temperature is
approached or exceeded in use, naturally
the mechanical properties will be
consequently affected.
Short
time
strength values at temperatures up to
350°C are approximately 90% that of the
room temperature strength.
Quenched and tempered Martensitic
Stainless Steels show a marked drop in
Toughness as the temperature is lowered
to 0°C and below.
However, if the steels have been tempered at the highest tempering temperatures (650°-700°C) these initially lowered
impact values are of the order of 15-40 J
Izod V. With due care and cognisance,
their use may be considered at low temperatures. However, their Toughness
progressively decreases as the temperature falls, and at 150°C below freezing
they become exceedingly brittle.
CONCLUSION
The Mechanical Properties of the three
classifications of the Family of Stainless
Steel have been covered in a general
manner.
How they are measured, and the factors
which govern and influence their values
are similar for the other classifications
of Stainless Steel, and in fact, for steel
in general.
The range of attainable values for the different mechanical properties is vast, and
this is more particularly so for the
Austenitic Stainless Steels. This factor is
but one of those which renders Stainless
Steel an extremely versatile material.